1. Field of the Invention
This invention generally relates to a method and apparatus for investigating earth formations traversed by a borehole, and more particularly, pertains to a new and improved method and apparatus for electromagnetic well logging. Still more particularly this invention relates to a new and improved method and apparatus for acquiring multiple signals during induction logging, combining these signals in software, and creating a depth tagged representation of resistivity or conductivity from a weighted sum of signals from that depth or from that depth and neighboring depths.
2. Description of the Prior Art
Generally speaking, in electromagnetic well logging, commonly referred to as "induction logging," a transmitter coil energized by alternating current is lowered into a well or borehole and indications are obtained of the influence of surrounding formations on the electromagnetic field established by the coil. Usually such indications are obtained by observing the voltage induced in a receiver coil lowered into the borehole in coaxial relationship with the transmitter coil and axially spaced apart therefrom.
Commonly available induction logging tools include a transmitter array and a receiver array each consisting of a set of coils connected together in series. Such a transmitter/receiver array is illustrated in U.S. Pat. No. 3,067,383 issued Dec. 4, 1962 to Dennis R. Tanguy. The transmitter is driven by a constant amplitude current at 20 kHz. The induction measurement consists of recording the total voltage induced in the receiver array.
Such commonly available commercial logging tools in service today record only the component of the total voltage which is 180 degrees out of phase (called the "in-phase" or "real voltage, R") with the transmitter current. Such an induction sonde is sensitive to a spatial average of the conductivity of the surrounding formation. The averaging function is determined by the placement and winding of the transmitter and receiver coils.
Logging methods and apparatus are described in U.S. Pat. No. 4,513,376 issued Apr. 23, 1985 to Thomas Barber where not only is determined the real "R" or in-phase of the receiver voltage component, but also the quadrature or 90.degree. out-of-phase "X" component of the receiver voltage. The R and X components are used to estimate formation resistivity.
Only minimum processing of the receiver data is currently used with commercially available tools to enhance receiver voltage signals. Tools designed for medium radial measurement of conductivity use a single point by point boost method to account for skin-effect in the formation. Such a "boosting" of the receiver signal is described in U.S. Pat. No. 3,226,633 issued Dec. 28, 1965 to W. P. Schneider.
Tools designed for deep radial measurement of conductivity use a three depth deconvolution process followed by a skin effect boost. Such a deconvolution process is described in U.S. Pat. No. 3,166,709 issued Jan. 19, 1965 to Henri-Georges Doll. The purpose of the three point deconvolution is to reduce the tool response sensitivity to high conductivity shoulders which are far away.
Another approach to induction logging is described in a paper by D. W. Martin presented to an SPWLA Symposium in 1984 entitled, "The Digital Induction--A New Approach to Improving the Response of the Induction Measurement". Martin describes a "Digital Induction Sonde" which includes four receiver coils spaced vertically above a transmitter coil on a sonde mandrel The signal induced in each of the four receiver coils is amplified, digitized and transmitted direct to the surface for computer processing before the resistivity is determined for the borehole log.
All the "raw data" from the receiver coils is recorded and stored for computer processing. Specifically, the depth of investigation can, during post job processing be varied continuously, thereby allowing an invasion profile to be produced.